1. Technical Field
The invention relates to planar-magnetic loudspeakers and more particularly to a push-pull, planar-magnetic loudspeaker allowing field replacement of the diaphragm.
2. Description of the Problem
FIG. 1 illustrates a double-ended, or push-pull, magnetic-planar transducer 20, conventional in the prior art. This structure is characterized by two magnetic arrays 10 and 11 supported by perforate substrates 14, 24 positioned on opposite sides of a flexible diaphragm 12, which includes a conductive coil 13. The film is tensioned into a planar configuration. An audio frequency drive signal is supplied to the coil 13, and a variable voltage and current thereby provided in the coil gives rise to a variable magnetic field, which interacts with the fixed magnetic field set up by and between the magnet arrays 10 and 11. The diaphragm is displaced in accordance with the audio frequency drive signal, thereby generating a desired acoustic output. An example representing this art area is found in U.S. Pat. No. 4,156,801 issued to Whelan.
Because of a doubled-up, front/back magnet layout of the prior art push-pull magnetic structures, double-ended systems have been generally regarded as more efficient, but also as more complex to build. Double-ended designs are also particularly sensitive to deformation from repulsive magnetic forces that tend to deform the devices outward. Outward bowing draws the edges of the diaphragm closer together, and alters the tension of the diaphragm. This can seriously degrade performance and, over time, can render the speaker unusable necessitating disposal or repair of the unit.
Assembly of push-pull structures presents its own difficulties. As observed in U.S. Pat. No. 6,934,402 issued to Croft III, et al., the magnetic circuits of the front and back magnetic structures interact. The repulsion forces make it difficult to have a stable mechanical structure, while providing a focused field and making better use of magnetic material than single-ended devices. Assembly of push-pull structures presents its own difficulties. Like magnetic poles are brought into close proximity, requiring substantial force to bring the assembly together.
In U.S. Pat. No. 5,850,461 it was further observed that in the field of planar magnetic acoustic transducers and related flat type transducers utilized for sound reproduction, the tensioning of the sound producing diaphragms is important to obtain uniform sound reproduction. The proper tensioning of a diaphragm within a support frame has created difficulty in manufacturing processes. In planar magnetic transducers, if sufficient diaphragm tensioning is not provided, the diaphragm may vibrate into one of the stators formed by spaced magnets mounted within a support frame in spaced relationship with respect to an electrical circuit applied to the diaphragm. Improper tensioning throughout the diaphragm also results in changes in the vibrational characteristics of the diaphragm throughout its “sound producing” or “active” area which is generally that area defined internally of the diaphragm support frame.